There is such a thing as low-mercury tuna — albacore!
This is the best choice, allowing you to maximize the health benefits of high omega-3 fats, yet minimize contaminants like mercury and reduce unnecessary bycatch. Choose the most sustainably harvested type, caught by troll/pole from Canadian and U.S. Pacific waters. It's available year-round fresh, frozen, canned, or smoked.
Albacore tuna is low in fat, high in protein, with not only an abundance of heart healthy omega-3 fatty acids but antioxidant rich selenium, too. It's low in mercury and other pollutants because the fish are caught at a young age, before they accumulate high levels of toxins. The Environmental Defense Fund says we can safely eat up to four servings a month of albacore tuna.
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Steer clear of bluefin tuna. This species is large, fast, and tasty, but rare, like endangered species rare. Because Atlantic bluefin often sells for more than a thousand dollars a kilogram, it's being pushed ever closer to the brink of extinction. (So, don't be that guy.)